1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driver circuit. More particularly, the present invention relates to an LED driver circuit utilizing a voltage clamping device.
2. Description of the Related Art
LED backlight for television (TV) and liquid crystal display (LCD) for personal computers (PCs) are gaining popularity because LED backlight enables thinner display and saves power. The driver circuit for an LED backlight module generally drives multiple strings of LEDs. Each string includes multiple LEDs connected in series.
For example, FIG. 1 is a schematic diagram showing a constant-current conventional LED driver circuit 100. A boost pre-regulated stage (not shown) steps up an input voltage (not shown) to the bus voltage VBUS. The driver circuit 100 drives eight LED strings 111-118. The driver circuit 100 includes eight sections and each section drives one of the LED strings 111-118. For example, the section driving the LED string 118 includes the operational amplifier (OPA) 138, the N-channel metal-oxide-semiconductor field-effect transistor (N-MOSFET) 128, and the resistor 148. Each section of the LED driver circuit 100 is identical in function and circuit structure.
The brightness of an LED is in direct proportion to the current flowing through it. For consistent brightness and display quality, current matching among the LED strings is important. Due to factors such as fabrication variation, it is impossible for multiple LED strings to have exactly the same forward voltage when turned on. Therefore, a controlling mechanism is necessary to implement current matching of the LED strings.
Take the LED string 118 for example. The N-MOSFET 128, the OPA 138, and the resistor 148 constitute a control loop of negative feedback, which regulates the current through the resistor 148 and the LED string 118 to a predetermined value I118. I118=Vref/R148. As long as the reference voltage Vref is stable and the resistors 141-148 have well-matched resistances, the currents through the LED strings 111-118 are substantially the same. Please notice that N-MOSFET 128 and OPA 138 are operating like a low-drop-out (LDO) linear regulator.
Further, the LED driver circuit 100 provides multi-level dimming of the LEDs. The dimming level of the LEDs is controlled by the dimming signal PWMD. FIG. 2 is a waveform diagram showing the dimming signal PWMD, the output of the OPA in each LED-string-driving section, and the current through each LED string in the LED driver circuit 100. During the time periods T1-T2 and T3-T4, the dimming signal PWMD is high. The OPA output to the gate of the corresponding N-MOSFET is turned on. The corresponding N-MOSFET is turned on accordingly. The OPA regulates the current through the corresponding N-MOSFET and the LED string to the predetermined value. During the time periods T2-T3 and T4-T5, the dimming signal PWMD is low. The OPA output to the gate of the corresponding N-MOSFET is turned off. The corresponding N-MOSFET is turned off accordingly. The current flowing through the LED string falls to zero. The average LED current is proportional to the duty cycle of the dimming signal PWMD. That is, Iave=D*Ion. Lave is the average LED current and Ion is the full stabilized LED current when the corresponding N-MOSFET 121-128 is turned on. The duty cycle is defined as, D=Ton/(Ton+Toff)=(T2−T1)/(T3−T1). Therefore, by varying the duty cycle of the dimming signal PWMD, one can control the average LED current lave, thus, the effective brightness, of the LED strings 111-118.
Presently, most LEDs used for backlight applications are 20 mA devices. The forward voltage VF of an LED operating at 20 mA ranges from 3.0V to 3.8V over the temperature range of −20° C. to 80° C. and the manufacturing tolerance. And presently, there are many 6-channel and 8-channel LED backlight drivers from various IC vendors. For example, Maxim's MAX8790A, Texas Instruments' TP S61181, and Intersil's ISL97636A are 6-channel drivers. MAX17061 and Linear Technology's LT3760 are 8-channel drivers. Most of these 6-channel or 8-channel drivers have built-in N-MOSFETs with Vdss rating of 40V to 45V. In general, each channel can drive a sting of up to 10 LEDs in series connection. Therefore, an 8-channel LED backlight driver can drive up to 80 LEDs at the most.
A 42-inch LCD-TV typically uses 800 to 1200 LEDs for its backlight system. Therefore, it needs ten to fifteen such 8-channel drivers.
However, using so many drivers will complicate the current matching performance. This is due to the reference voltage Vref may vary among those driver chips. Further, too many driver chips add the wiring complexity and the backlight module (BLU) cost. Therefore, it would be ideal to substantially increase the number of LEDs per channel, thus to reduce the number of driver chips and wiring complexity. But unfortunately, the number of LEDs each channel can support is limited to the voltage rating of the N-MOSFET.
FIG. 3 is a schematic diagram showing the output breakdown of a conventional N-MOSFET with a rated output breakdown voltage Vdss of 40 volts. The horizontal axis in FIG. 3 is the drain-to-source voltage Vds of the N-MOSFET, while the vertical axis in FIG. 3 is the drain-to-source current Ids of the N-MOSFET. With a gate-to-source voltage Vgs of 0, the N-MOSFET exhibits the current-voltage characteristics curve 310. With a gate-to-source voltage Vgs of 2 volts, the N-MOSFET exhibits the current-voltage characteristics curve 320.
When an N-MOSFET connected to an LED string is turned off (the gate-to-source voltage Vgs=0V) and the bus voltage VBUS is higher than the output breakdown voltage Vdss of the N-MOSFET, the drain-to-source voltage Vds of the N-MOSFET will rise toward the bus voltage VBUS until the N-MOSFET breaks down. Once the N-MOSFET breaks down, its drain-to-source current Ids rises quickly. How high the breakdown current will reach depends heavily on the semiconductor process and the N-MOSFET device structure. In general, the N-MOSFET's behavior in the breakdown region is erratic and highly unpredictable. In extreme situations, it may result in destructive device failure. The bus voltage VBUS is substantially equal to the combined forward voltage VF of the LED string. At the same time, the bus voltage VBUS should not exceed the rated output breakdown voltage Vdss in order to prevent breakdown. Therefore, the combined forward voltage VF of the LED string should not exceed Vdss. In other words, the number of LEDs in an LED string is limited by the rated output breakdown voltage Vdss of the corresponding N-MOSFET.